Methods of modifying a video signal are used to correct the frequency response and/or improve the image definition of a screen display. Video-signal changes with small or medium amplitude are to be emphasized by the correction signal to accentuate fine image details. Large amplitude changes of the video signal, however, are not to be emphasized or steepened, since this would result in an unnatural three-dimensional effect, and also the resulting large amplitude of the modified video signal may cause defocusing in the case of fine details known as blooming.
Methods of modifying video signals are known in the prior art, U.S. Pat. No. 4,573,075 entitled DIGITAL SIGNAL CORING APPARATUS WITH CONTROLLABLE CORING THRESHOLD LEVEL, and issued to Bolger, describes a digital signal processing apparatus for providing a coring threshold controllable in response to at least a portion of the digital signal to be cored. A digital coring circuit removes samples of digital input signals in a range determined by a digital threshold level signal. The threshold level signal is developed by a coring control device which makes use of a digital reference level. The device is used in processing digital television signals in a television receiver.
German Patent Application No. DE 39 19 817 A, entitled CIRCUIT FOR VIDEO SIGNAL DIGITAL APERTURE CORRECTION discloses a device wherein a digital aperture correction signal is derived from a video signal. The signal is modified by a circuit with adjustable, non-linear characteristics. The modified signal is added to the video signal. The non-linear characteristic provides aperture correction signal rise from zero to specified values according to correction signal values.
U.S. Pat. No. 4,270,026 entitled INTERPOLATOR APPARATUS FOR INCREASING THE WORD RATE OF A DIGITAL SIGNAL OF THE TYPE EMPLOYED IN A DIGITAL TELEPHONE SYSTEM, issued to Shenoi et al., and assigned to ITT Corporation, describes a device for increasing the sampling or word rate of a digital input signal. Conversely, U.S. Pat. No. 4,302,631, entitled DECIMATOR APPARATUS FOR DECREASING THE WORD RATE OF A DIGITAL SIGNAL OF THE TYPE EMPLOYED IN DIGITAL TELEPHONE SYSTEMS, issued to Shenoi et al., and also assigned to ITT Corporation, describes an apparatus to reduce the word rate and increase the word length of an out put digital signal of an analog to digital converter without conventional hardware multipliers. Such devices are commonly used in signal processing circuits for digitized video signals.
Another known possibility of generating a correction signal, in which a modified video signal has at least some of the above-mentioned characteristics, is to transmit the correction signal unchanged up to a predetermined amplitude, or with a predetermined attenuation or gain, by performing an amplitude-limiting action, and, if this predetermined amplitude is exceeded, to transmit a constant, maximum amplitude.
Particularly in digital systems, such a transfer function of the amplitude limiter can have a disadvantageous effect, since the correction signal being limited contains distortions in the form of harmonics which, depending on the modulation of the transfer function used, and on the ratio of desired signal frequency range to sampling frequency, may be folded back into the desired signal frequency range of the correction signal and produce interfering alias components there.
It is therefore an object of the present invention to provide an apparatus and method for generating a modified video signal wherein the interfering signals resulting from the amplitude-limiting action performed on the correction signal are minimized.